1. Field of the Invention
This invention relates to digital image-processing apparatus for introducing synthesized high-frequency components in a variably-enlarged size of a displayed two-dimensional (2D) image, wherein (1) any undesired visible artifact in the displayed image is effectively eliminated and (2) the apparatus may be efficiently implemented on an integrated-circuit (IC) chip.
2. Description of the Prior Art
Known in the art are various digital-processing techniques for enlarging the size of a displayed image by introducing interpolated pixel values between pixel values of the original input image. Unfortunately, the introduction of such interpolated pixel values inherently results in the widening of image edge information, thereby causing the problem of undesirable blurring of edges in the displayed enlarged image. In order to overcome this problem, the prior art has developed different ways of synthesizing image components having higher spatial frequencies than the highest spatial frequency of the original input image, which synthesized image components are then added to the displayed image. In this regard, incorporated by reference herein are the teachings of U.S. Pat. No. 5,717,789 issued to Anderson et al. on Feb. 10, 1998, which is entitled xe2x80x9cImage Enhancement By Non-Linear Extrapolation in Frequency Spacexe2x80x9d, and its U.S. divisional Pat. No. 6,005,983 issued on Dec. 21, 1999.
More particularly, Anderson et al. strips off image 2D high frequencies using a low-pass filter (LPF) and subtraction process. The high pass component is then subjected to a non-linear function that amplifies the individual Laplacian values and clips or bounds them to a predefined threshold. Finally, this modified high pass component image is added back to the original image, or as an extension further expanded and interpolated by two and then added back to a matching expanded original image. The non-linear function creates modified amplitude values that reflect great slope at the zero-crossings of the Laplacian, thus synthesizing higher-order frequency terms. Their method is a rather simple non-adaptive approach that can readily be implemented in integrated circuit form, or software form for non-real-time applications. However, this Anderson et al. approach for synthesizing higher-order frequency terms inherently results in introducing undesired visible non-linear artifacts in the displayed enlarged image. This severely limits its utility, particularly in the processing of JPEG and MPEQ compressed images.
Further incorporated by reference herein are the teachings of our U.S. patent application Ser. No. 09/112539 filed Jul. 9, 1998, entitled xe2x80x9cSynthetic Bandwidth Extrapolation for Imagesxe2x80x9d, which is assigned to the same assignee as the present application. In particular, this patent application discloses a non-linear, computationally-intensive digital process for deriving zooming images which employs local statistics within the image in order to optimally sharpen the image at each located image edge by adding a synthesized component having a higher spatial frequency than the highest spatial frequency of that edge in the original input image.
Additionally incorporated by reference herein are the teachings of our U.S. Pat. No. 5,355,328 issued Jul. 9, 1998, entitled xe2x80x9cResampling Apparatus Suitable For Resizing a Video Imagexe2x80x9d, which permit an original input image to be either reduced or, alternatively, enlarged size in each of two spatial dimensions (2D) by a non-integer fractional factor.
In view of the foregoing discussion of the prior art, there is a need for a non-linear, relatively computationally-simple digital process for deriving zooming images which does not suffer from the introduction of undesirable visible non-linear artifacts in the displayed enlarged image, thereby producing a perceptually natural looking displayed enlarged image that retains the xe2x80x9clook and feelxe2x80x9d of the original input image.
The present invention is directed to improved structure for an image zoom module of a digital image processing apparatus that comprises at least one such image zoom module. The image zoom module is responsive to a digital two-dimensional (2D) Gaussian input image having a certain pixel density in each dimension thereof and a first frequency bandwidth applied thereto for deriving a digital 2D Gaussian output image having twice the certain pixel density in each dimension thereof and a second frequency bandwidth which is wider than the first frequency bandwidth. The improved structure comprises (1) 2D high-pass filter means responsive to pixels of the Gaussian input image being applied as an input thereto for deriving pixels of a first Laplacian image therefrom that has the certain pixel density in each dimension thereof and the first frequency bandwidth; (2) a first diagonal interpolator responsive to the pixels of the first Laplacian image being applied as an input thereto for deriving pixels of a second Laplacian image therefrom that has twice the certain pixel density in each dimension thereof and the first frequency bandwidth; (3) a phase gapper responsive to the pixels of the first Laplacian image being applied as an input thereto for deriving pixels of a binary-mask output therefrom that has the certain pixel density in which only an edge location in the first Laplacian image is defined by a pair of adjacent pixels of the binary-mask output having given non-zero amplitude values and all other locations in the first Laplacian image are defined by pixels of the binary mask output having zero amplitude values; (4) 2D expand means including 2D low-pass filter means responsive to pixels of the binary-mask output being applied as an input thereto for deriving pixels of a 2D low-passed filtered output image therefrom having twice the certain pixel density in each dimension thereof; (5) pixel-processing means comprising a multiplier for multiplying the algebraic values of corresponding pixels of the second Laplacian image from the first diagonal interpolator and the 2D low-passed filtered output image from the 2D expand means for deriving a third Laplacian image having the second pixel density and the second frequency bandwidth; (6) a second diagonal interpolator responsive to pixels of the Gaussian input image having the first pixel density and the first frequency applied thereto for deriving a Gaussian image having the second pixel density and the first frequency bandwidth as an output therefrom; and (7) a summer for adding the algebraic values of corresponding pixels of the third Laplacian image having the second pixel density and the second frequency bandwidth from the pixel-processing means and the Gaussian image having the second pixel density and the first frequency bandwidth from the second diagonal interpolator to derive the corresponding pixels of the 2D Gaussian output image having twice the certain pixel density in each dimension thereof and the second frequency bandwidth as the sum output from said summer.